I bought two mbed development kit (Wifi Dipcortex), when I connect them to my windows 8.1 USB port, none of them is recognized. It seems windows can detect them but it cannot find a correct driver to install for them. I really appreciate your help in this matter.

Windows 8.1 is a pain when it comes to USB. Microsoft have further locked down USB drivers for "Security" which means by default it wont allow unsigned drivers. But you can get around it. Google "windows 8.1 unsigned USB drivers" one such guide is here : http://www.howtogeek.com/167723/how-to- ... d-drivers/